True face of climate's hockey stick graph revealed

IT'S all about speed – and quicker isn't better. One of the most extensive analyses yet of past temperatures shows that the world is warming faster than at any time in the last 11,300 years.

The study has produced the first detailed extension of the famous – and sometimes contentious – "hockey stick" temperature graph right back to the end of the last ice age. It suggests that Earth is not quite out of its natural range of temperature variation yet, but will be by the end of the century.

The finding comes as atmospheric carbon dioxide levels made their second-biggest leap on record. Monitoring stations in Hawaii recorded them rising by 2.67 parts per million between January and December 2012. The biggest jump was in 1998, when they rose by 2.93 ppm. Last year they averaged 393.81 ppm, up from 315.97 ppm in 1959 when records began. Climatologists estimate that CO2 needs to plateau at 450 ppm for a 50 per cent chance of avoiding dangerous warming of 2 °C or more.

Building a high-resolution picture of how temperatures have changed is no mean feat. Thermometre readings only go back to around 1860, so we have to use proxies to delve further back. Tree rings, for instance, are thicker during warm years when trees can grow faster.

Shaun Marcott of Oregon State University in Corvallis and colleagues used 73 proxies to reconstruct temperatures throughout the Holocene – the epoch that began 11,300 years ago, after the last ice age. Their analysis shows that over thousands of years, temperatures rose and fell by less than 1 °C. "It took 8000 years to go from warm to cold," says Marcott. Agriculture, communal life and societies with various forms of government all arose during this relatively stable period.

The rate of recent warming is unlike anything that happened in at least 11,000 years, says Michael Mann of Pennsylvania State University in University Park, who created the original hockey stick graph (see "inset"). Rapid change is the real issue of warming, says Mann, because it challenges our ability to adapt in time.

The gradual drift before the 19th century was driven by changes in Earth's axis of rotation, says Marcott: the planet's tilt increased early in the Holocene before decreasing again. "It sort of wobbles," Marcott says. A greater tilt leads to more sunlight at the poles in summer, and this keeps the planet warmer.

If humans had not begun warming the planet by releasing greenhouse gases, Earth would eventually return to an ice age. "If we were following the orbital trend we'd still be cooling," Marcott says.

Marcott's figures suggest that the planet is now nearly as warm as at its warmest point in the last 11,000 years. Some climatologists have claimed that it is already even hotter, but for now, uncertainties in the data make it difficult to say for sure.

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